This invention relates to the field of prosthetic orthopaedic devices and assemblies and, in particular, to a novel orthopaedic cement.
All joints in the human body are subject to destruction by disease and trauma. The goal of skeletal prostheses is permanent, functional replacement of bone and joints which have failed. Polymeric materials have shown great promise as orthopaedic implants. In fact, one of the more successful orthopaedic implants is the total hip system where the acetabular component is ultra High molecular weight polyethylene (UHMWPE) and the femural component is a metallic alloy. Acrylic cement is generally utilized in the fixation of the prosthesis components.
Acrylic cements were first developed for dental uses about 30 years ago and were subsequently modified for orthopaedic use. These cements are not adhesive. They function by mechanical interlocking with surrounding porous bone structure.
Composition of Acrylic Bone Cement: The composition of a typical acrylic cement would be:
______________________________________ % by Powder Liquid Component Volume Component % by Weight ______________________________________ methyl methacrylate 97.5 polymethyl 88 methacrylate* N,N--dimethyl 2.5 benzoyl per- 2 toluidine oxide Hydroquinone 75 ppm barium sulfate 10 ______________________________________ *or mixture of polymethyl methacrylate (PMMA) with copolymers of methyl methacrylate (MMA) and minor amounts of styrene of other methacrylate monomer.
The cement is supplied to the surgeon in kit form which typically contains a 20 ml ampule of sterile monomer liquid and a 40 gm pouch of sterile polymer powder. It should be noted that for proper handling characteristics the polymer powder is generally composed of spherical particles or mixtures of spherical particles with irregularly shaped particles. The particle size is usually less than 200 mesh with diameters typically in the 10 to 30 micron range.
During surgery the cement is prepared by mixing the liquid and powder components. Within a few minutes (4-5) the cement becomes dough-like and is ready, at this point, to be worked into the bone cavity. The prosthesis is then inserted and aligned. The cement hardens in about 10 to 15 minutes to fix the device in place. The time intervals between the various stages of cement consistency depend on the particular product and ambient conditions at the surgical site.
After polymerization, or "hardening", the cement contains approximately 2 to 5% residual monomer and has considerable entrapped air. The powder particles are retained so that the polymerized cement is actually a composite material wherein PMMA particles are dispersed in a newly polymerized PMMA matrix. The physical properties of existing prosthetic acrylic cements are lower than those of conventional PMMA polymer.
______________________________________ Acrylic Commercial Cement PMMA ______________________________________ compressive str, psi 9,000-14,000 11,000-19,000 tensile str, psi 3,600-6,000 8,000-10,000 tensile modulus, psi 2.3-3.8 .times. 10.sup.5 3.5-5.0 .times. 10.sup.5 ______________________________________
In the total prosthesis system, the cement functions as a boundary between the prosthesis and the bone and in this role greatly improves the load bearing capacity of the prosthesis compared with the condition without cement.
The modulus of the femural component (metal) is approximately 15 to 35 million psi. Cement modulus is approximately 230,000 to 380,000 psi, and the modulus of the cancellous bone adjacent to the cement is approximately 10,000-70,000 psi. The order of decreasing modulus is Em to Ec to Eb. This situation then dictates that complex dynamic stresses (and strains) generated during normal body functions are transmitted through the prosthesis to the cement and ultimately to the bone.
The major long term complication of such prosthetic work is loosening of the prosthesis. Such failure frequently begins to appear 3 to 5 years after surgery.
Currently, an unconstrained ultra-high molecular weight polyethylene (UHMWPE)/metal alloy prosthesis, fixed with acrylic cement, offers the most resistance to loosening over other prosthesis systems. However, even with these improved systems, loosening still remains the most prevalent cause of joint replacement failure.
Loosening can occur in any one of the three areas:
1. Prosthesis/Cement PA2 2. Cement/Cement PA2 3. Bone/Cement
1. Failure at the prosthesis/cement interface occurs when relative motion exists between these two components. In the total hip system this is often seen with the femural metal component. PA1 2. Failure within the cement is due to fracture of the cement. This type of behavior has been noted with acrylic cement. PA1 3. Failure at the bone/cement interface is the most common cause of loosening and, in part, may be traced to the behavior of living tissue in direct contact with a foreign body (implant). Bone is extremely stress sensitive: too little stress or too much stress will cause bone resorption (bone retreating from the interface with cement) leading to loosening. Between the limits of too little or too much, intermittent stress will provoke bone formation. The stress generated in the prosthesis and transmitted through the cement to the bone/cement interface will determine the reaction of the bone to the prosthesis. If the stress is well distributed and is within physiological limits bone will be formed and retain vitality. PA1 Glass transition temperature below 37.degree. C. PA1 Non-tacky polymer particles PA1 Proper solubility in MMA PA1 diethylene glycol diacrylate or dimethacrylate tetraethylene glycol diacrylate or dimethacrylate polyethylene glycol diacrylate or dimethacrylate trimethylolpropane triacrylate or trimethacrylate Bisphenol A diacrylate or dimethacrylate ethoxylated Bisphenol A diacrylate or dimethacrylate pentaerythritol tri- and tetraacrylate or methacrylate tetramethylene diacrylate or dimethacrylate methylene bisacrylamide or methacrylamide dimethylene bisacrylamide or methacrylamide N,N'-dihydroxyethylene bisacrylamide or methacrylamide hexamethylene bisacrylamide or methacrylamide decamethylene bisacrylamide or methacrylamide divinyl benzene. PA1 benzoyl peroxide PA1 lauroyl peroxide PA1 methyl ethyl peroxide PA1 diisopropyl peroxy carbonate
Although acrylic bone cement does help to distribute the load, therefore lowering the overall stress level, this load distribution is not necessarily uniform and high stress points can occur particularly in the region of the lesser trochanter and the calcar. The cancellous (plate or honeycomb-like) bone functions quite effectively as a skeletal shock absorber in its natural state. Furthermore, the shock absorbing abilities of cancellous bone are totally dependent upon the bending and deformation of the trabecular (plate) arrangement. Filling the narrow space between trabecular bone with a high modulus material like acrylic cement seriously stiffens the trabecular bone and decreases the ability of the trabecular plates to bend and buckle. This, coupled with the high stresses transmitted to the bone/cement interface, often results in boney fracture which results in loosening of the prosthesis.
In summary, clinical observations have shown that the majority of total joint replacement failures (excluding failures due to poor surgical technique or infection) are directly or indirectly related to the functional deficiencies of acrylic cement previously known to the art.
Several approaches have been proposed by previous workers to increase fracture resistance, and prolong fatigue life.
An approach taken to improve the fracture behavior of the cement has been the incorporation of fibers. However, the reported studies in this area have revealed few instances where the fracture resistance has been significantly improved. On the other hand, this approach invariable brings about an increase, not a decrease, in the cement modulus. Furthermore, the fibers cause a drastic reduction in the cement's flow characteristics which can result in a poor mechanical interlock with the bone in actual use.
Still another suggestion has been to try to lessen the cement's residual stresses. These stresses arise from the shrinkage of the material during the polymerization and subsequent cooling in situ. When such stresses exist, the cement will internally fracture under lower loads than it would otherwise.
It is to be realized that this discussion of the background of the invention is necessarily made with a full knowledge of the invention to be disclosed below and is not meant to be construed as a view of the prior art as it may be construed by one having no prior knowledge of this invention.